1. Field of the Invention
The present invention relates to an optical element, especially a wavelength dispersion element used in an optical communication system, an optical measuring system, etc.
2. Related Art
It is known well that photonic crystals having a structure in which dielectric materials different in refractive index are arranged periodically at intervals of about a wavelength of light has the following properties;
(a) confinement of light by photonic band gaps;
(b) very large wavelength dispersion due to a unique photonic band structure; and
(c) abnormality in group velocity of propagated light.
Various optical elements using these properties have been proposed.
Photonic crystals can be classified into three groups by the number of directions having periodic structures as follows:
(i) one-dimensional photonic crystals;
(ii) two-dimensional photonic crystals; and
(iii) three-dimensional photonic crystals.
For example, the simplest one-dimensional photonic crystal is a dielectric multilayer filter formed in such a manner that two kinds of thin films (e.g., SiO2 and TiO2) are laminated alternately on a parallel-plane substrate. The dielectric multilayer filter has been already put into practical use. Because this structure has photonic band gaps in the periodic direction, this structure has a function of reflecting only incident light of a specific wavelength region. Furthermore, because the wavelength region in the photonic band gap with respect to oblique incident light varies according to the direction of polarization, this structure can be made to function as a polarized light separating filter.
Although the one-dimensional photonic crystal has a large merit that easy to produce, the one-dimensional photonic crystal has not been investigated as much as two-dimensional and three-dimensional photonic crystals because there are few methods making good use of the properties of photonic crystals except the aforementioned filter. The one-dimensional photonic crystal, however, can use the property of “very large wavelength dispersion due to a unique band structure” or “abnormality in group velocity of propagated light” sufficiently though the one-dimensional photonic crystal is inferior in the function of “confinement of light by photonic band gaps” to two-dimensional and three-dimensional photonic crystals. As means using the property, there is an example in which an end surface of the multilayer film, particularly, a surface of the exposed multilayer structure, is used as a light input surface or as a light output surface.
For example, theoretical analysis of the direction of light rays incident onto an inclined section of the multilayer film has been described in Applied Physics B, Vol.39, p.231, 1986. There has been disclosed the fact that the same polarized light separating effect as in a birefringent material can be obtained by use or the property (so-called structural birefringence) that the refractive index of the multilayer film varies widely according to whether the polarization is TE polarization or TH polarization, with intention of separating polarized light by structural birefringence (Optics Letters, Vol.15, No.9, p.516, 1990). There has been further a report that very large dispersion (super-prism effect) can be obtained because the shape of the first photonic band of the periodic multilayer film is linear in a neighbor of a band gap (“International Workshop on Photonic and Electromagnetic Crystal Structures” Technical Digest, F1–3).
A structure in which air holes are arranged in a thin film on a substrate by application of photolithography has been already examined well as a structure of two-dimensional photonic crystal. If a linear defect in formed in the arrangement of the air holes, the portion of the linear defect can be provided as an optical waveguide.
If the photonic band gap is provided throughout all directions in three-dimensional photonic crystal, a three-dimensional waveguide can be provided. Accordingly, there is expectation that a large number of optical elements can be incorporated in an element about 1 mm square when three-dimensional photonic crystal in used.
A spectroscopic element using photonic crystal is also called super-prism. Very large wavelength dispersion can be obtained compared with a general prism or diffraction grating. For example, Physical Review B, Vol.58, No.16, p.R1096, 1998 has reported an experimental result that angular dispersion per 1% wavelength difference in use of three-dimensional photonic crystal amounts to tens of degrees. When, for example, a material large in wavelength dispersion is used as a spectroscopic element for separating a signal containing different wavelengths in wavelength division multiplexing (WDM) communication, the size of the device as a whole can be reduced to be very small.
Incidentally, when photonic crystal is to be used as a practical spectroscopic element, there are several problems.
Parallel light flux is spread at a certain angle by a diffraction phenomenon. The spread of light flux becomes wide as the light flux becomes thin. It is therefore preferable that the light flux is thick sufficiently to obtain a spectroscopic element with high wavelength resolving power, Conversely, if the light flux is thin, even a spectroscopic element large in angular difference due to wavelengths cannot exhibit high resolving power.
To secure such thick light flux, the size of the element must be large to a certain degree. In addition, it is inevitable that the length of the optical path of light propagating in the element becomes long. It is however difficult to produce a large element in the case of two-dimensional or three-dimensional photonic crystals. Furthermore, attenuation of light in the elements often exhibits a large value. Accordingly, it is difficult to form a practical spectroscopic element.